Phthalic anhydride is an important commercial chemical useful in the manufacture of plasticizers, polyesters, alkyd resins and dyes.
Phthalic anhydride is typically produced from raw materials such as orthoxylene (o-xylene), petroleum naphthalene, and coal-tar naphthalene. The price of these raw materials and, as a direct result, the price of phthalic anhydride have fluctuated greatly depending upon supply and demand. Because the cost of the raw materials is a major factor in the price of phthalic anhydride, it is of great importance that any system used to produce phthalic anhydride capture as much of the resultant product as possible.
Phthalic anhydride can be successfully produced from any of a number of processes, i.e., (1) air oxidation of o-xylene in fixed-bed reactors, (2) air oxidation of petroleum or coal tar naphthalene in fixed-bed reactors, (3) fluid bed oxidation of o-xylene, (4) fluid bed oxidation of petroleum or coal tar naphthalene, and (5) liquid phase oxidation of o-xylene or naphthalene.
The general process scheme for the various vapor phase routes is to mix the hydrocarbon feed (in the vapor form) with compressed air and to feed the mixture to fixed-bed reactors which contain tubes packed with catalysts, e.g., vanadium oxide and titanium dioxide coated on an inert, nonporous carrier. When fluid bed reactors are used, the hydrocarbon feed in liquid form can be injected directly into the fluidized bed so that the air and the hydrocarbon are mixed in the reactor to produce a reactor effluent gas (i.e., the vapor phase oxidation product). The reactors are equipped with means for removing the heat of the oxidation reaction. The heat that is removed is used to generate steam.
After the vapor phase oxidation product exits either the fixed-bed or fluid bed reactors, it is cooled to cause the phthalic anhydride to condense. This allows separation of the phthalic anhydride from the gas stream. The phthalic anhydride is typically condensed as a solid. However, a two-stage condensation system can be used to first condense a portion of the phthalic anhydride as a liquid and then to condense the remainder as a solid.
Expensive switch condensers that operate alternatively on a cooling cycle and a heating cycle are used to collect the phthalic anhydride as a solid. The solid is then melted for removal from the condensers.
The use of switch condensers to separate crude phthalic anhydride from a vapor phase oxidation product is described in U.S. Pat. No. 5,214,157 (Healy et al.), issued May 25, 1993, which is incorporated herein by reference. The resultant vapor phase oxidation product is cooled close to the solidification point (131.degree. C.) of phthalic anhydride and any condensed liquid is usually separated out before the remaining vapor enters the switch condensers. The switch condensers desublime the vapor phase oxidation product using the cold condenser oil, and then melt off the solid phase crude phthalic anhydride product using a hot condenser oil heated with steam. Both the hot condenser oil and cold condenser oil are pumped through switch condensers via horizontally disposed heat exchange tubes.
A substantial amount of impurities exit switch condensers as part of the vapor stream, whereas the crude phthalic anhydride product is plated out on the heat exchange tubes as a solid during the cooling step and exits the switch condensers at the bottom as a liquid during the melting step. This crude phthalic anhydride liquid is collected from the switch condensers in surge vessels before being pumped to storage for crude finishing to commercial product. The vapor gases from the switch condensers are sent to waste gas incinerators where the by-products are destroyed by oxidation to carbon dioxide and water. This can be done in combination with fuel gas to produce steam.
Unfortunately, switch condensers involve a significant portion of the capital and operating costs of a phthalic anhydride plant. The cost of each switch condenser, including installation, can exceed a million dollars. Also, switch condensers operate in a batch mode on 3-6 hours cycles to desublime solid phthalic anhydride on the heat exchange tubes.
The present inventor has developed a unique process scheme which avoids the need to use expensive switch condensers in order to recover the phthalic anhydride from the vapor phase oxidation product. This unique process continuously condenses and recovers phthalic anhydride in the liquid phase without the formation of an intermediate solid phase.
The continuous liquid recovery process of the present invention provides the following advantages over conventional switch condensers: (1) fewer pieces of processing equipment; (2) continuous versus batch mode of operation; (3) recovery of more than 99.7% of the phthalic anhydride from the vapor phase oxidation product versus 99-99.4% for switch condensers; (4) typical losses of 0.25 to 0.5% of the crude phthalic anhydride production in the light ends distillation following recovery by switch condensers are significantly reduced due to the recycle of the light ends cut; (5) since the process concentrates the by-products from a low vapor concentration (e.g., for the maleic anhydride from less than 0.1 mol % in the vapor to more than 50 mol % in the recovered liquid streams), the maleic and citraconic anhydrides and the benzoic acid by-products can be readily further concentrated and upgraded for commercial sales of these by-products; (6) the cost of maleic anhydride recover for sale is lower than for typical plants because the impurities such as citraconic anhydride can be rejected into the recycled impure maleic anhydride and eventually purged with the benzoic acid stream and (7) benefits the environment because the waste gas contains less by-products and less phthalic anhydride.
In addition, this process has advantages over solvent recovery processes in that only materials already present are used for the recycle. No new material is added. In addition to cost of a solvent, some of the solvent will escape to the environment. Also the solvent could adversely effect the product quality of the phthalic anhydride or recovered by-products. Even if the recovery step using an ester is added to this recovery process, only the alcohol portion of the molecule is extraneous to the process stream because the acid portion of the ester is made from one of the acids and/or anhydrides in the process stream.